Hypertension is the most important risk factor for cardiovascular (CV) diseases and remains the number one cause of morbidity and mortality for our country. Despite the variety of traditional antihypertensive agents available, the blood pressure of about 40% of patients is still difficult to manage. A major component of drug resistant hypertension is neurogenic hypertension with increased vasomotor/cardiac sympathetic drive and decreased parasympathetic tone. Many studies demonstrate the importance of the brain renin-angiotensin system (RAS) in the development of neurogenic hypertension. Recently, a new component of the brain RAS was discovered and named the (pro)renin receptor (PRR).
PRR promotes Angiotensin II (Ang II) generation and activates both Ang II-dependent and -independent signaling pathways through binding to renin and prorenin. It is now well recognized that Ang II is produced and acts locally in the central nervous system (CNS) and serves a crucial role in CV function. So far, the beneficial effects of RAS blockade have been attributed to the inhibition of the vasoconstriction and hypertrophy-inducing properties of Ang II. Thus, PRR provides a new target to study the effect of the brain RAS in hypertension because it is a novel component of the RAS, controlling the production of the vasoconstrictor Ang II and the hypertrophic signaling pathways through both Ang II-dependent and -independent signaling pathways.
Current antihypertensive agents target RAS components. Examples include angiotensin-converting enzyme (ACE) inhibitors, Ang II type 1 receptor (AT1R) blockers, and direct renin inhibitors. However, all of these compounds cause dramatic increases of plasma renin levels due to the negative feedback loop (decrease of Ang II levels) on renin production. Renin and prorenin directly bind to PRR and can activate signaling pathways independent of Ang II. The clinical relevance of PRR is particularly significant in situations where there are increases in renin and prorenin levels.
The activation of PRR initiates an intracellular signaling pathway involving mitogen-activated protein kinase which increases the synthesis of profibrotic molecules such as plasminogen activator inhibitor-1, fibronectin, collagen and transforming growth factor-β. These signaling pathways have been shown to directly link to diabetic retinopathy, nephropathy, cardiac hypertrophy, vascular and kidney fibrosis. In addition, prorenin, an activating ligand of PRR, is found at levels one hundred times higher in the plasma of a diabetic patient than the amount of prorenin found in healthy individuals. Several studies indicate that PRR expression was increased in diabetic retinopathy, nephropathy, and in hypertension.
Some peptides have been developed to antagonize prorenin for use in diabetic nephropathy, high salt induced hypertension in mouse and rat models. However, the effects of these peptides remain controversial because several other independent laboratories were not able to replicate the effects of this peptide.
Thioether bridges have been used in modifying small peptides in the past to increase stability. However, there are currently no available compounds or thioether bridge-modified peptides for treating hypertension that act on the (pro)renin receptor.